ESSENTIAL QUESTION: What threats do nuclear weapons pose to the world?

For decades, tensions have mounted between North Korea and the U.S. North Korea’s leaders view the U.S. as a danger to their government and have long threatened to attack America if provoked. Until recently, though, North Korea was unable to back up its threats. The nation began developing nuclear weapons in the early 2000s, but it wasn’t until this July that it tested its first intercontinental ballistic missile (ICBM)—one that experts believe could reach the mainland U.S.

Bad feelings have grown between North Korea and the U.S. for decades. North Korea’s leaders believe the U.S. is a danger to their government. For a long time, they’ve threatened to attack if America gives them a reason. North Korea couldn’t back up its threats until lately. It began developing nuclear weapons in the early 2000s. But the nation didn’t test its first intercontinental ballistic missile (ICBM) until this July. Experts believe this missile could reach the mainland U.S.

North Korea claims it needs nuclear weapons to protect itself. The U.S. and other countries, however, argue that North Korea’s repressive and isolated regime can’t be trusted with such dangerous weapons. President Donald Trump has said that the U.S. is prepared to take military action if Kim Jong Un, North Korea’s leader, doesn’t stop testing nuclear weapons.

In September, North Korea stated that Trump’s comments were a “declaration of war.” Could a nuclear exchange between North Korea and the U.S. really happen, and if so, what’s at stake? Science World spoke with experts on the issue. Here are the facts on some of the most pressing scientific questions about North Korea and its nuclear program.

North Korea says it needs nuclear weapons to protect itself. But the U.S. and other countries say that North Korea can’t be trusted with such dangerous weapons. That’s because its government is secretive and restricts freedom. U.S. President Donald Trump warned North Korea’s leader, Kim Jong Un, to end his country’s nuclear weapons testing. If he doesn’t, Trump said that the U.S. is ready to take military action.

In September, North Korea said that Trump’s words were a “declaration of war.” Could a nuclear battle between North Korea and the U.S. really happen? If so, what’s at stake? Science World spoke with experts working to understand the problem. They answered some of the biggest questions about North Korea’s nuclear program.

WHAT’S A NUCLEAR WEAPON?

A nuclear weapon is a bomb that uses nuclear energy—power from the reactions caused when atoms and atomic particles collide—to create an explosion, says Ferenc Dalnoki-Veress. He is a physicist at the James Martin Center for Nonproliferation Studies in California. He explains that there are two main types of nuclear weapons: atomic bombs and hydrogen bombs.

Atomic bombs rely on nuclear fission—a process that occurs when an uncharged particle called a neutron splits apart the nucleus, or center, of an atom. Atomic bombs split atoms of the chemical elements uranium (U) or plutonium (Pu), producing a massive amount of energy.

When atoms and atomic particles crash into each other, a reaction happens. It releases power called nuclear energy. A nuclear weapon is a bomb that uses nuclear energy to create an explosion, says Ferenc Dalnoki-Veress. He’s a physicist at the James Martin Center for Nonproliferation Studies in California. There are two main types of nuclear weapons, he explains. They are atomic bombs and hydrogen bombs.

Atomic bombs use nuclear fission. In this process, an uncharged particle called a neutron splits an atom’s nucleus, or center. Atomic bombs split atoms of the chemical elements uranium (U) or plutonium (Pu). This produces a huge amount of energy.

The same fission reaction happens in a hydrogen, or thermonuclear, bomb. The reaction triggers nuclear fusion. That’s when two atoms fuse together. The hydrogen bomb contains hydrogen isotopes—different forms of an element. These isotopes combine and release energy (see Fission vs. Fusion).

WHY ARE NUCLEAR WEAPONS A CAUSE FOR CONCERN?

Nuclear weapons are extremely destructive. During World War II, U.S. President Harry S. Truman made the decision to drop the world’s first atomic bombs—one on Hiroshima, Japan, and a second on Nagasaki, Japan. Each instantly killed tens of thousands of people.

Nuclear weapons cause extreme damage. U.S. President Harry S. Truman decided the U.S. would drop the world’s first two atomic bombs during World War II. They fell on Hiroshima, Japan, and Nagasaki, Japan. Each killed tens of thousands of people instantly.

The devastation didn’t end with those blasts. The elements used in nuclear weapons are highly radioactive. This means that they are unstable and emit high-energy particles and waves. After the explosions, this radioactive material spread for hundreds of miles. The resulting nuclear fallout sickened people and animals, poisoned natural resources like water, and tainted food sources. More than 100,000 additional people died.

Nine countries are known to currently possess nuclear weapons (see Nuclear Nations). But unlike the rest of these countries, North Korea has made numerous threats to use its weapons. North Korea is also very secretive—there’s no way to know if the country gives or sells its weapons to other countries or to terrorist groups.

The damage from the nuclear bombs didn’t end with the blasts. The elements used in nuclear weapons are highly radioactive. They’re unstable and give off high-energy particles and waves. After the explosions, this radioactive material spread for hundreds of miles. This nuclear fallout made people and animals sick. It also poisoned natural resources like water and polluted food sources for years. More than 100,000 additional people died.

We know that nine countries have nuclear weapons (see Nuclear Nations). But only North Korea has made many threats to use its weapons. The country is also very secretive. No one can know if North Korea gives or sells its weapons to other countries or terrorist groups.

HOW DO WE KNOW THAT NORTH KOREA HAS NUCLEAR WEAPONS?

Nuclear bombs are complex devices. Countries often test them before adding the weapons to their arsenals. “Luckily, we’re going to know about it if someone tests one,” says Melissa Hanham, a researcher at the James Martin Center.

“The first indication of an underground nuclear test is seismic,” says Hanham. “There are many stations that monitor vibrations from earthquakes and volcanoes, and they also pick up nuclear tests.” The huge explosions appear on devices called seismographs as small, artificial earthquakes.

Nuclear bombs are complex devices. Countries often test them first. Then they add the bombs to their stores of weapons. “Luckily, we’re going to know about it if someone tests one,” says Melissa Hanham. She’s a researcher at the James Martin Center.

“The first indication of an underground nuclear test is seismic,” says Hanham. “There are many stations that monitor vibrations from earthquakes and volcanoes, and they also pick up nuclear tests.” The explosions appear on devices called seismographs. These explosions look like small, human-made earthquakes.

On September 3, these stations detected seismic activity near North Korea’s nuclear test site. By looking at those reports, satellite imagery, and other sources, Hanham and her colleagues deduced that the seismic activity was produced by a nuclear blast 10 to 20 times more powerful than the bomb dropped on Hiroshima. Although this is weaker than the U.S.’s most advanced weapons, it was North Korea’s strongest test yet.

On September 3, these stations picked up seismic activity near North Korea’s nuclear test site. Hanham and her co-workers studied those reports, satellite pictures, and other sources. They found that the seismic activity came from a nuclear blast. It was 10 to 20 times more powerful than the bomb dropped on Hiroshima. This was North Korea’s strongest test yet.

WHAT’S THE RANGE OF NORTH KOREA’S MISSILES?

Over the summer, North Korea successfully tested two ICBMs. Experts believe these missiles could travel 10,400 kilometers (6,462 miles). That puts large parts of the U.S. within their range (see In Range?). But North Korea can’t necessarily hit the U.S. with a nuclear weapon just yet, says Dalnoki-Veress. “You need to have both the right delivery system and a small enough weapon to put on that delivery system,” he says. “The smaller the weapon, the harder it is to make.”

Over the summer, North Korea successfully tested two ICBMs. Experts believe these missiles could travel 10,400 kilometers (6,462 miles). Large parts of the U.S. are within their range (see In Range?). Does that mean North Korea can hit the U.S. with a nuclear weapon? Maybe not, says Dalnoki-Veress. “You need to have both the right delivery system and a small enough weapon to put on that delivery system,” he says. “The smaller the weapon, the harder it is to make.”

Dalnoki-Veress says that it’s uncertain whether or not North Korea has made a small enough nuclear weapon to fit on an ICBM. Even if the country does succeed, the threat of retaliation by the U.S. or its allies would hopefully prevent North Korea from ever using the weapon—a concept called nuclear deterrence. If deterrence fails, there are still missile defense systems designed to shoot down enemy rockets.

We don’t know if North Korea has made a nuclear weapon that small, Dalnoki-Veress says. It would have to fit on an ICBM. If the country does succeed, it might be afraid to use the weapon. That’s because the U.S. or its allies could strike back. This idea is called nuclear deterrence. What if deterrence fails? Missile defense systems are ready to shoot down enemy rockets.

WHAT’S BEING DONE TO REDUCE THE NUCLEAR THREAT?

The U.S. and its allies continue to pressure North Korea to stop developing nuclear weapons. This approach involves military exercises and sanctions, or economic penalties, against North Korea and countries that trade with it. So far, North Korea has only responded with new weapons tests and more threats.

The U.S. and its allies keep pushing North Korea to stop developing nuclear weapons. One way they do this is with military exercises. Another is sanctions. These are economic penalties against North Korea and countries that trade with it. But North Korea has only answered with new weapons tests and threats.

The tension between North Korea and the U.S. isn’t the first time two nuclear powers have faced off. During the Cold War—a period between the end of World War II in 1945 and 1991—the U.S. and the Soviet Union (part of which is now Russia) were sometimes only moments from all-out nuclear war. Thankfully, common sense and diplomacy prevailed.

Arms control and disarmament—the reduction and eventual elimination of nuclear weapons—are hard to accomplish when fear rules the day. “But there’s a powerful role for students to play moving forward,” says Hanham. “Understanding the science can help us measure and reduce risk—and make the right decisions for the world.”

This isn’t the first time two nuclear powers have faced off. It happened during the Cold War, a time between the end of World War II in 1945 and 1991. Then the trouble was between the U.S. and the Soviet Union (part of which is now Russia). Sometimes, the two powers were only minutes away from all-out nuclear war. But common sense and diplomacy won out.

Today, many people argue for arms control and disarmament. They want nations to reduce and finally get rid of nuclear weapons. But it’s hard to do when fear rules the day. “There’s a powerful role for students to play moving forward,” says Hanham. “Understanding the science can help us measure and reduce risk—and make the right decisions for the world.”